Conventional display elements incorporated in display devices may be categorized into electro-optical elements having, for example, their luminance or transmittance controlled through an applied voltage and electro-optical elements having, for example, their luminance or transmittance controlled through a current flow. Typical examples of the former electro-optical elements include liquid crystal display elements, and those of the latter electro-optical elements include organic EL elements, which are also called OLEDs (organic light-emitting diodes). The organic EL display device, incorporating organic EL elements (i.e., self light-emitting electro-optical element), more readily allows for reduced thickness, low power consumption, and high luminance designs and provides other benefits than the liquid crystal display device which requires a backlight and color filters. Organic EL display devices have been actively developed in recent years for these reasons.
The organic EL display device is driven by a passive matrix method (also called a simple matrix method) or an active matrix method, both of which are well known. The organic EL display device driven by a passive matrix method has a simple structure, but is difficult to increase screen size and achieve high definition. In contrast, the organic EL display device driven by an active matrix method (hereinafter, an “active-matrix organic EL display device”) is easier to increase screen size and achieve high definition than the organic EL display device driven by a passive matrix method.
An active-matrix organic EL display device includes a matrix of pixel circuits formed therein. Each pixel circuit in the active-matrix organic EL display device typically includes an input transistor through which a pixel is selected and a drive transistor through which a current supply to the organic EL element is controlled. Note that in the following description, the current flow from the drive transistor to the organic EL element may be referred to as the “drive current.”
FIG. 23 is a circuit diagram of a configuration of a conventional, typical pixel circuit 91. The pixel circuit 91 is provided at every intersection of data lines S and scan lines G in the display unit. As shown in FIG. 23, the pixel circuit 91 includes two transistors T1 and T2, a capacitor Cst, and an organic EL element OLED. The transistor T1 serves as an input transistor, and the transistor T2 serves as a drive transistor.
The transistor T1 is disposed between the data line S and the gate terminal of the transistor T2. The transistor T1 has its gate terminal connected to the scan line G and its source terminal connected to the data line S. The transistor T2 is disposed in series with the organic EL element OLED. The transistor T2 has its drain terminal connected to a power supply line that feeds a high-level power supply voltage ELVDD and its source terminal connected to the anode terminal of the organic EL element OLED. The power supply line that feeds the high-level power supply voltage ELVDD will hereinafter be referred to as the “high-level power supply line.” The high-level power supply line will be denoted by the same reference sign “ELVDD” as the high-level power supply voltage. The capacitor Cst has one of its terminals connected to the gate terminal of the transistor T2 and the other terminal connected to the source terminal of the transistor T2. The organic EL element OLED has its cathode terminal connected to a power supply line that feeds a low-level power supply voltage ELVSS. The power supply line that feeds the low-level power supply voltage ELVSS will hereinafter be referred to as the “low-level power supply line.” The low-level power supply line will be denoted by the same reference sign “ELVSS” as the low-level power supply voltage. Also, in this description, the node where the gate terminal of the transistor T2, a terminal of the capacitor Cst, and the drain terminal of the transistor T1 are connected will be referred to as a “gate node” for convenience and denoted by a reference sign “VG” Usually, either the drain or the source that has a higher potential is termed the drain. In this specification, however, one of them is defined as the drain, and the other as the source; the source potential may be in some cases higher than the drain potential.
FIG. 24 is a timing chart depicting an operation of the pixel circuit 91 shown in FIG. 23. The scan line G is not selected until time t01. Therefore, until time t01, the transistor T1 is off, thereby maintaining the gate node VG at an initial potential level (e.g., a level that is in accordance with the writing in the preceding frame). At time t01, the scan line G is selected, thereby turning on the transistor T1. Accordingly, a data voltage Vdata that corresponds to the luminance of the pixel (subpixel) formed by the pixel circuit 91 is supplied to the gate node VG via the data line S and the transistor T1. Thereafter, the potential at the gate node VG changes in accordance with the data voltage Vdata in the period up to time t02. In that period, the capacitor Cst is charged to a gate-to-source voltage Vgs which is a difference between the potential at the gate node VG and the source potential of the transistor T2. At time t02, the scan line G is deselected. In response to this, the transistor T1 is turned off, establishing the gate-to-source voltage Vgs to be maintained by the capacitor Cst. The transistor T2 feeds the organic EL element OLED with a drive current that matches the gate-to-source voltage Vgs maintained by the capacitor Cst. As a result, the organic EL element OLED emits light at a luminance that matches the drive current.
The organic EL display device typically includes thin film transistors (TFTs) as drive transistors. However, thin film transistors tend to vary in threshold voltage from one to the other. The numerous drive transistors in the display unit, having various threshold voltages, will cause variable luminance, which degrades display quality. In addition, the drive transistor and the organic EL element exhibit voltage-current characteristics that degrade over time, thereby allowing a current flow to decrease over time even under the same applied voltage. The luminance of the organic EL element therefore decreases gradually over time. Furthermore, the organic EL element has a light emission efficiency that decreases over time. Therefore, even if the organic EL element is supplied with a fixed current, its luminance decreases over time. These phenomena cause image sticking. To address these issues, processes have been suggested and implemented that compensate for the variations and adverse changes of the threshold voltage of drive transistors and the degradation (including decreases of the light emission efficiency over time) of organic EL elements.
It is noted here that in relation to the present invention, some documents are known as will be briefly described here. PCT International Application Publication, No. WO2014/208458 discloses an invention of an organic EL display device in which properties (characteristics) of both the drive transistor and the organic EL element are detected so that the organic EL element can be supplied with a drive current that compensates for both the degradation of the drive transistor and the degradation of the organic EL element. Japanese Unexamined Patent Application Publication, Tokukai, No. 2012-83777 discloses an invention of a light-emitting device in which changes that occur in the electrode potential of a monitoring element (light-emitting element provided for the purpose of monitoring) as a result of temperature changes and device degradation over time are fed back to the light-emitting elements to maintain the luminance of the light-emitting elements at a fixed value. Japanese Unexamined Patent Application Publication, Tokukai, No. 2009-80252 discloses an invention of an organic EL display device in which the signal amplitude reference voltage (the voltage that is in the video signal amplitude and determines the black level) and the signal value reference voltage for determining an amplitude for a signal value fed to a pixel circuit are varied with sensed temperature to compensate for temperature-dependent luminance variations while maintaining high image quality.